Abstract/Executive Summary Skin cancer is the most common cancer ? more common than all other cancers combined. Despite public health efforts, melanoma rates are still increasing. The proposed research addresses this important problem using an innovative approach: using online advertising and social media to target groups at risk of skin cancer. Additionally, it addresses health disparities by focusing on gay men, who are an understudied, high-risk population. Finally, our novel approach allows for unprecedented reach at relatively low intervention cost. Our team has extensive experience with observational and interventional behavioral research using social media as well as significant experience working with sexual minority men. Our central hypothesis is that targeted online prevention messages can influence knowledge, attitudes, and behaviors about tanning among gay men. The rationale underlying this research is that one of the groups at highest risk of developing melanoma from tanning bed exposure ? gay men ? is engaged on social media, so this is the platform to best reach them. In Aim 1, we will identify which melanoma prevention messages are most effective for gay men. Using focus groups, cognitive interviews, and online response data, we will determine the most persuasive messages that motivate gay men to avoid tanning beds. In Aim 2, we will assess the impact of melanoma prevention messages on knowledge and behaviors relevant to melanoma risk and on self-reported intent to use tanning beds, which is a validated proxy for tanning behavior. We will first optimize the delivery of our prevention messages by assessing how timing (season and time of day), frequency, and number of messages impact engagement with a prevention message: including number of views and clicks that direct to a website with melanoma prevention information. We will then test the effect of prevention messages by performing a rapid, randomized, controlled trial on Facebook (using the most effective messages identified in Aim 1) placed as advertisements. The intervention group and control group will each include 3,600 gay men, for a total study population of 7,200 gay men. These specific aims have been designed to provide the essential feasibility and preliminary efficacy data to support future large-scale and long-term research on the impact of targeted online prevention messages on hard outcomes, such as reductions in actual tanning bed use and decreases in state-specific melanoma incidence.